1. Field of the Invention
The present invention is related to multi-layer printed wire boards (PWBs), such as for use in electronic devices, and more particularly to multi-layer PWBs for use in mobile terminals.
2. Description of Related Art
Multi-layer printed wire boards (PWBs) are the platform on which complex electronic components such as integrated circuits and a number of passive components, such as capacitors and resistors, are mounted. PWBs serve as a structural foundation and a hub for electrical connections for a variety of electronic devices. Specifically, multi-layer PWBs allow for a plurality of interconnected conductive layers to be packed into a compact space, and as such, they are useful in manufacturing portable electronic devices, including mobile terminals. Multi-layer PWBs must provide not only a compact hub for electrical connections but also a robust mechanical and electrical connection between the electronic components that make up a given device. Of particular interest in portable electronic devices is increasing the drop reliability of the PWB, or the ability of the PWB to maintain a physical and electrical connection between electronic components even after being subjected to the mechanical shock from a drop onto a hard surface, as users of portable electronic devices are unfortunately prone to do.
Originally, drop reliability of multi-layer PWBs was increased by simply increasing the overall thickness and stiffness of the multi-layer PWB. While this solution is effective in securing the mechanical and electrical connections between components on the PWB upon a mechanical shock, it has the negative result of making the miniaturization of the PWB more difficult. In this regard, portable electronic devices are continually being made smaller and, as such, it is desirable that the constituent components, such as the PWB, be similarly made smaller. In addition, thicker and stiffer multi-layer PWBs often suffer from decreased reliability under thermal load, since solder reliability and electrical connectivity under varying thermal load is decreased as the thickness of the multi-layer PWB is increased. This decreased reliability is due in part to the inability of the thick and stiff PWB to flex and conform to the subtly changing sizes and shapes of electrical components and their solder connections during thermal load cycles.
One conventional PWB that is utilized in mobile telephones has eight copper layers separated by dielectric layers or resin coated copper layers. Beginning from one surface of the PWB, a first outermost copper layer is disposed upon a resin coated copper layer which, in turn, is disposed upon a second copper layer. The second copper layer is disposed on a first dielectric layer which, in turn, is disposed on a third copper layer. The third copper layer is disposed upon a second dielectric layer which, in turn, is disposed upon a fourth copper layer. The fourth copper layer is disposed upon a third dielectric layer. The third dielectric layer is centrally located within the PWB and the PWB structure is effectively mirrored about the third dielectric layer. As such, the third dielectric layer is disposed upon a fifth copper layer which, in turn, is disposed upon a fourth dielectric layer. The fourth dielectric layer is disposed upon a sixth copper layer which, in turn, is disposed upon a fifth dielectric layer. The fifth dielectric layer is disposed upon a seventh copper layer which, in turn, is disposed upon a second resin coated copper layer. The second resin coated copper layer is disposed upon an eighth copper layer which forms the opposed surface of the PWB.
Typically, the dielectric layers are formed of a FR-4 glass fiber/epoxy material, such as an FR-4 glass fiber/epoxy material bearing the designation MCL-E-679F provided by Hitachi, Ltd. Additionally, the resin coated copper layers may be formed of a material bearing the designation MCF-6000E that is also provided by Hitachi, Ltd.
The copper layers may be electrically connected by means of vias through the resin coated copper layers and/or the dielectric layers. Based upon the various electrical connections and the components mounted upon the first and eighth copper layers, the PWB can therefore provide the desired functionality.
While this conventional PWB generally performs as desired, this PWB is thicker and stiffer than desired. In this regard, the first, second, seventh and eighth copper layers of a conventional PWB have a thickness between 25 um and 50 um with a nominal thickness of 35 um, while the third, fourth, fifth and sixth copper layers have a thickness of between 12 um and 19 um with a nominal thickness of 17 um. Additionally, the resin coated copper layers of a conventional PWB have a thickness of between 50 um and 70 um with a nominal thickness of 60 um, while each dielectric layer is quite thick and contributes substantially to the overall thickness of the PWB with a thickness between 125 um and 175 um and a nominal thickness of 150 um. In this regard, the thickness of the PWB contributes to the overall size of the mobile terminal and it would therefore be desirable to reduce the size of the PWB and, in turn, the size of the mobile terminal. Additionally, this conventional PWB has not performed as desired in terms of drop reliability. In other words, the PWB has a tendency to no longer function properly after a lesser number of drops than is desired. Since consumers are demanding increased reliability in portable electronic products, it is also desirable to improve the drop reliability of the PWB.
Therefore, it would be advantageous to have an optimized multi-layer PWB structure with an increased mechanical strength and drop reliability, while still maintaining a thin and flexible structure that is compact and less susceptible to connection failure under cyclical thermal loads.